It has been proposed to form the outer shell of an exhaust muffler for an internal combustion engine with synthetic resin material having favorable anti-corrosive and sound-insulating properties while the inner pipe which is required to be heat-resistant is made from metallic material. As a mode of combining these two components made of different materials, it would be convenient to form the outer shell from two halves, and interpose the inner tube between two halves of the outer shell.
However, according to such a structure, the materials of the inner pipe and the outer shell are so different in material properties, such as the coefficient of thermal expansion, the coefficient of heat transfer, and elastic modulus, that the parts joining these two components tend to be subjected to significant thermal stress because the inner pipe is directly exposed to the heat of the exhaust gas and has a greater coefficient of thermal expansion than the outer shell. In particular, when these two components are rigidly joined together by using such an adhesive agent as thermo-setting resin, the outer shell which is lower than the inner pipe in mechanical strength may develop cracks, or the adhesive agent may be detached from the outer shell after repeated thermal stress. Further, the heat from the inner pipe will adversely affect the parts joining the two halves of the outer shell together.
Japanese utility model laid open publication No. 02-85819 discloses a substantially cylindrical outer shell for an exhaust muffler combining a heat resistant metallic or ceramic tube and an anti-corrosive and sound-insulating synthetic resin sleeve member. The end plates of the outer shell are each provided with a deformable portion for accommodating both radial and axial relative displacement between the outer shell and the inner pipe. However, this structure requires a large number of component parts, and requires time-consuming steps which are not suited for automotization for its assembly. Therefore, this conventional exhaust muffler would be unacceptably expensive to manufacture.